Due to its simplicity, the usage of binary signaling to transmit data from one integrated circuit to another is virtually universal. Depending upon the binary value of the bit being transmitted, a transmitting integrated circuit either charges its output terminal to a positive signaling voltage or to ground. The output terminal couples over a transmission line to an input terminal at the receiving integrated circuit. The receiving integrated circuit determines whether its input terminal was charged or discharged to determine the binary value of the received bit.
Although binary signaling is advantageously simple to implement, the transmitting integrated circuit must not only charge its output terminal but also the transmission line coupling it to the input terminal on the receiving device. If the transmission line is relatively short, its capacitance may not be too significant. But as the transmission line length grows, the capacitance rises with it. The power consumption of cycling such a capacitive load between the positive signaling voltage and ground may thus become prohibitive, particularly for mobile devices that must conserve battery charge.
The power consumption is not only a function of the transmission line capacitance but also depends on the magnitude of the positive signaling voltage. For example, it is conventional to use a power supply voltage as the positive signaling voltage. Assuming that a positive logic convention is used, the transmission of a binary one bit thus requires the transmission line to be charged “full rail” to the power supply voltage. To reduce the associated power consumption, it is known to configure the transmitter such that it does not use the power supply voltage as the positive signaling voltage but instead uses some lower voltage value. For example, it is known to use an NMOS pull-up transistor having a current source connected to its source. The current source interacts with the NMOS pull-up transistor so that the positive signaling voltage is the power supply voltage minus the threshold voltage for the NMOS pull-up transistor. Although this lowers the output voltage swing, it comes at the expense of the power dissipated by the current source.
Accordingly, there is a need in the art for low-power transmitters having an output voltage swing that is lower than the power supply voltage.